The three-time winner took an armchair ride to the finish in Victor Harbour on the back of team-mate Daryl Impey to claim the win and 10 valuable bonus seconds.
The Orica-GreenEDGE team captain finished ahead of Ben Swift (Sky) and Giacomo Nizzolo (Trek-Segafredo) to all but lock down the race ahead of its Queen Stage on Willunga.
"I'm absolutely thrilled with that. I think I've caught a few people by surprise, myself included," Gerrans said.
"I just had a really good run into the finish. What the team were able to do today set me up for that first intermediate sprint and then again for the final. They're really making my life as easy as possible."
Gerrans also picked up an extra second over Jay McCarthy (Tinkoff) by beating him earlier in the first intermediate sprint.
The victory came after a three-rider break containing Alexis Gougeard (AG2R), David Tanner (IAM) and Patrick Shaw (UniSA) were caught with 20km left to race on the 138km stage from Norwood.
The trio built a lead of almost six minutes before Orica GreenEDGE lifted the tempo to reduce the margin, but any hopes of victory for the leading three were later quashed as BMC and Tinkoff joined the chase.
Gerrans now leads Jay McCarthy by 14 seconds and Rohan Dennis by 26 seconds ina race that is often won by narrower margins.
The 151.5km fifth stage from McLaren Vale to Willunga will decide the race and Gerrans will be more watchful than aggressive in his approach to the stage.
“I still have to be very very attentive going into Willunga tomorrow," he said. We have a nice little buffer, but there is still a heck of a lot of work to be done.
“The jersey has changed hands many times up Willunga hill so we will still have our work cut out for us.
“It’s not in my interest to attack at all, I’ve really got to try to defend this jersey. It’s up to the other guys to drop me and get a bit of time on the climb now but the way my team is riding and supporting me, it gives me a lot of confidence going into tomorrow.”